One of the more common mistakes a student makes in stoichiometric
calculations is interpreting the chemical reaction as a mass
rather than mole relation. For example, consider the oxidation
of iron metal with molecular oxygen:

A beginning student might incorrectly interpret this equation
from the perspective of a mass relation: 2 grams of iron reacts
with 3 grams of oxygen to produce 1 gram of iron(III) oxide.
Clearly this interpretation is wrong as it would result in a
violation of the principle of conservation of mass. The correct
interpretation if this equation is the mole-mole relation: